Hollow Earth Conspiracy – This is one is difficult to classify because it has characteristics of a conspiracy theory but also can be classified as mythology.
For thousands of years, many cultures have had stories about a hollow Earth, and it shows up in Greek and Roman mythology as well as Mayan and Aztec mythology. Most of these myths speak of an opening to the underworld in which people can enter and exit. It implies that you can travel to the underworld if you just know where the opening is located. Thus enters the conspiracy part of it.
Supposedly there are groups of people that know where the opening is located but keep it a secret. During World War II, the Germans spent a lot of time looking for the opening to the hollow earth. They seemed to believe it was in the South Pole, and they even built a base there. Some people believe that near the end of the war, some of the highest ranking Nazis the disappeared were actually taken to this secret base in the South Pole. They retreated under ground to live there in a new society in the center of the Earth.
There are other theories that believe that demons live in the center of the Earth and can enter and exit through the secret opening to the surface world. This one is probably biblically inspired, and it is the least popular in current times.
The ancient Greeks had a concept of the underworld. It was ruled by Hades, god of the underworld and later thought of the god of the dead. Much of the mythology from this period associated the underworld with the dead. The concept of Hell may have evolved from the concept of Tartarus which was a deep gloomy part of the underworld described as the dungeon of the underworld.
The reality is that most of the planet is rich with caves and caverns of different types and sizes. Anciant Europeans often lived in caves. These caves were often in the side of mountains and may have been level or even pitched in an upward direction. They do not seem to be the type of caves that could be mistaken for a hollow Earth. In South America there are caves systems that run across the entire continent and are often hundreds of miles long. Some are filled with water, but many are dry cave systems. These caves often have enormous caverns, and there is archeological evidence that they were used by ancient cultures. Many of these caves make their way deep into the Earth and have never been completely explored. It would be safe to say that the majority of these cave are unknown to modern cultures or at least haw been explored little. These caves are more likely to have inspired the hollow earth myths and conspiracies simply because so little is know about them.
For every myth and conspiracy, there may be a little truth.